STRUCTURE AND DEVELOPMENTAL RATE 117 



but no doubt the same conditions obtain among vertebrate ani- 

 mals in general, although in lower forms such differences in devel- 

 opmental results are more difficult to detect. 



The end products of development differ from one another to 

 varying degrees, slight differences are of little concern and are 

 classed as ordinary variations, but when the same deviations 

 become exaggerated they may be ranked as serious deformities or, 

 anomalies. This fact renders the analysis of normal develop- 

 mental processes and the experimental study of monstrous devel- 

 opment one and the same problem. It should be understood that 

 the present study is not intended solely as a contribution to so- 

 called 'teratology,' but is an experimental analysis and consid- 

 eration of the processes involved in all normal embryonic 

 development and growth. The experimental treatments have in 

 many of the cases caused the formation of well-known mon- 

 strous structures, but the point of importance is not the produc- 

 tion of the monster, but the simple alterations in the usual 

 course of events which have induced the modified structural 

 expression. 



For the past ten years I have claimed that all types of mon-/ 

 sters not of hereditary origin are to be interpreted simply as deJ 

 velopmental arrests. Such a position has been taken by otherst 

 (Dareste, '91). However, I propose at this time to present evi-\ 

 dence which clearly demonstrates the truth of the claim. By ^ 

 arresting development in very simple ways all types of monsters 

 may be obtained. The experiments have now reached such a 

 degree of exactness that the following propositions may be 

 stated as true, the evidence for which is recorded in the body of 

 this paper. 



First, all types of monster, double as well as single, may be 

 caused by one and the same experimental treatment ; second, any 

 one type of monster, such as cyclopia, may be produced by a 

 great number of different experimental treatments; third, all 

 effective treatments tend primarily to lower the rate of develop- 

 ment, and, fourth, the type of monster induced depends upon j 

 the particular developmental moment or moments during which 

 the developmental rate was reduced. Slowing the rate at one 



